If you go to the movies frequently, you've probably encountered THX at some point. The lights dim, the screen lights up and you see the THX logo, accompanied by earthshaking sound. The phrase "The audience is listening" appears and you know you are in for an aural, as well as visual, experience.

The common assumption is that THX is simply another movie sound format, but it turns out THX is not a sound format at all but something else entirely. In this edition of HowStuffWorks, you will learn just what THX is, how it works and how theaters acquire it.